1. Field of the Invention
The invention relates to ink jet recording apparatus having a single or a plurality of recording heads on a carriage and recording data by jetting ink droplets onto a recording medium from the recording head or heads. More particularly, the invention is directed to preliminary drive of the recording head.
2. Discussion of the Related Art
An ink jet recording apparatus that records data by applying heat to ink by a heater to produce bubbles and jetting the ink onto a recording medium from ink jetting openings of a head by the pressure derived from expansion of the bubbles has been developed.
In such ink jet recording apparatus, the ink is supplied to the front end of a nozzle section at all times. Thus, the ink at the nozzle section is dried at the time the apparatus is inoperative. To prevent the drying of the ink, a cap mechanism for covering the nozzle section is provided so that the nozzle section can be shut off from the outside air. However, even with the cap mechanism, it is difficult to maintain a constant degree of airtightness for a long time. As a result, the moisture and volatile component of the ink are gradually evaporated into the atmosphere from the vicinity of the ink jetting opening of the nozzle when left without covering or the like for a long period of time, changing its physical properties (particularly, its viscosity is increased), thereby making the ink hard to jet. If the normal printing operation is performed under such condition, the ink near the ink jetting opening of the nozzle is not jetted, or is jetted, however, in a direction deviating from the regular course under the normal nozzle drive conditions, thereby addressing the problem of defective printing, in both character and image.
This problem is encountered not only when the apparatus is left for a long period of time, but also when the apparatus is left under low temperatures. During printing, the temperature of the head is controlled optimally, so that stable printing can be ensured. However, as the recording apparatus is left inoperative for a long time, the temperature of the head is gradually decreased. A decrease in the temperature of the head, i.e., a decrease in the temperature of the ink increases the viscosity of the ink. As a result, even when the apparatus is left under lower temperatures, the same problem of poor printing in both character and image is addressed similarly to the case where the apparatus is left inoperative for a long time. In addition, the ink jetting amount is decreased, which causes the problem of inadequate image density or the like.
As is apparent from the above, it is ideal for the ink jet recording apparatus that the physical properties of the ink can be maintained stable when the apparatus is left inoperative over a long period of time. To achieve this, various methods of avoiding defective printing by attempting to improve head drive control have been developed. Proposed is a control involving the steps of driving the head on a preliminary basis by such a drive pulse as not to jet the ink before printing so that the apparatus can start printing data after the ink is ready to be jetted. For example, Japanese Patent Unexamined Publication No. Hei 4-28580 discloses a method of warming an apparatus up by using a pseudo print signal and a drive pulse, the pseudo print signal being a signal shorter than the genuine print signal. This method is not, however, applied to ink jet recording apparatuses.
Further, Japanese Patent Unexamined Publication No. Sho 62-179945 discloses a method of easily optimizing the preliminary drive operation by setting a drive pulse cycle and a drive pulse width of the recording head as the number of clocks and controlling the preliminary drive of the head based on the set number of clocks.
However, with these methods, the nozzle must be driven for a relatively large number of times to heat the head to a predetermined temperature under lower temperatures. Further, if the preliminary drive is effected with such a drive pulse as riot to jet ink as described above, the drive pulse width is limited to comparatively small values. This may increase the preliminary drive time.
It is also conceivable to shorten the time required for preliminary drive by decreasing the drive pulse cycle. In addition to the above-mentioned publications, Japanese Patent Unexamined Publication No. Hei 4-44856, e.g., discloses a method of decreasing the waiting time before the recording head is heated to a predetermined printable temperature after turning the power on by applying an electrical signal whose frequency is higher than the recording frequency. Such a method of using the higher frequency as the drive pulse allows quick and stable printing with shortened preliminary drive.
However, the above-mentioned method of using the drive pulse whose frequency is higher than the recording frequency not only burdens the recording head due to high frequency, but also elevates the cost of power with high current consumption by the power supply. The increase in the consumed current by the power supply imposes another problem if the number of recording heads mounted on the carriage is increased. Since the capacity of the power supply is limited in small recording apparatuses, the currently used power supply capacity may not be sufficient to meet control requirements in some cases. For this reason, preliminary drive that is optimal to an individual system configuration must be designed and effected.